Out-of-Plane Bending Testing of Chain Links

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life or 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A full scale test frame was constructed that has the capacity of applying inter-link rotation to a pre-tensioned chain. Although the test frame limits the number of links that can be tested together as a chain, a significant amount of testing was performed for the following chain sizes: 1. 81 mm Studded Grade R3S. 2. 107 mm Studdless Grade RQ3. 3. 124 mm Studless Grade R4. 4. 146 mm Studless Grade RQ4. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper the OPB mechanism is explained, and the test frame and results are presented. An empirical relationship is found to predict the OPB stresses in the chain links as a function of pretension and inter-link rotation. The OPB stress relationship obtained was applied to the failed mooring chain of the off-loading buoy with reasonable agreement. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

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Fatigue Testing of Out-of-Plane Bending Mechanism of Chain Links

Volume 3: Safety and Reliability; Materials Technology; Douglas Faulkner Symposium on Reliability and Ultimate Strength of Marine Structures ◽

10.1115/omae2006-92488 ◽

2006 ◽

Cited By ~ 2

Author(s):

Lucile Rampi ◽

Pedro Vargas

Keyword(s):

Fatigue Testing ◽

Empirical Relationship ◽

Fatigue Tests ◽

Full Scale ◽

Fatigue Performance ◽

Special Focus ◽

Out Of Plane ◽

Plane Bending ◽

Chain Links ◽

Mooring Chain

Three years ago, several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life or 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links (See Figure 1). This recently identified phenomena, Out-of-Plane Bending (OPB), explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail [3][4][5]. References [3] and [4] document full scale tests of the OPB mechanism using a full scale test frame with the ability of applying inter-link rotation to a pre-tensioned chain. This testing confirmed that interlink rotations with a constant tension load can result in significantly high stresses. OPB stresses were measured on four different chain sizes of various grades: 1) 81 mm Studded Grade R3S, 2) 107 mm Stud-less Grade RQ3, 3) 124 mm Stud-less Grade R4, and 4) 146 mm Stud-less Grade RQ4, Grade R3 in [3] and [4], but no actual fatigue tests were performed. References [3] and [5] document analytical and computational efforts to explain and quantify the OPB stresses. In this paper, special focus is placed on obtaining actual fatigue failures of chains from OPB loading. Smaller chain sizes (40 mm) are used to accommodate the load limits of the testing frame. To mimic the actual loading as close as possible, sub size models of actual chainhawses were used in the testing. Two chainhawses were used: 1) the chainhawse has internal curvature where a link rests on the intrados, similar to offloading buoy that failed in eight months, and 2) a straight chainhase, a design that is in use today with demonstrated improved fatigue performance over the curved chainhawse. OPB stresses are measured and reported. Fatigue loading in the OPB mode was applied for several configurations. The two chainhawse exhibit very different stress levels and fatigue performance. An empirical relationship previously reported in [3][4][5] is compared to the measured OPB stresses with mixed results. Although limited in number, the fatigue tests indicate that overall the chain fatigue performance is at or above the B1 DnV curve. The BS B1 curve is also compared.

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FEA of Out-of-Plane Fatigue Mechanism of Chain Links

24th International Conference on Offshore Mechanics and Arctic Engineering: Volume 3 ◽

10.1115/omae2005-67354 ◽

2005 ◽

Cited By ~ 4

Author(s):

Pedro Vargas ◽

Philippe Jean

Keyword(s):

Finite Element ◽

Fatigue Life ◽

Factor Of Safety ◽

Finite Element Models ◽

Chain Link ◽

Out Of Plane ◽

Bending Stresses ◽

Chain Links ◽

Mooring Chain ◽

Loading Procedure

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life of 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A previous paper [4] presented experimental results of applying inter-link rotation to a pre-tensioned chain. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper, the physics of the OPB mechanism is examined through finite element models of the 124mm chain link tests. The various modes of interlink rotation are examined. The proof loading procedure that the chain undergoes at manufacture is identified as a likely cause for creating a tightly mated surface that is conducive to activating the OPB mechanism. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

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Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) Static Test Program and OPB Interlink Stiffness

Volume 1: Offshore Technology; Offshore Geotechnics ◽

10.1115/omae2016-54195 ◽

2016 ◽

Cited By ~ 2

Author(s):

Lucile Rampi ◽

Fata Dewi ◽

Michel Francois ◽

Arnaud Gerthoffert ◽

Pedro Vargas

Keyword(s):

Fatigue Loading ◽

Time Frame ◽

Small Time ◽

Static Test ◽

Fatigue Design ◽

Out Of Plane ◽

Bending Stresses ◽

Plane Bending ◽

Mooring Chain ◽

Fatigue Mechanism

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the quasi static OPB stiffness measurement campaign and the post processing work to derive the OPB interlink stiffness.

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Fracture mechanics analysis for mooring chain links subjected to out-of-plane bending (OPB)

Marine Structures ◽

10.1016/j.marstruc.2020.102740 ◽

2020 ◽

Vol 71 ◽

pp. 102740

Author(s):

Xutian Xue ◽

Nian-Zhong Chen ◽

Yongchang Pu ◽

Xifeng Gao

Keyword(s):

Fracture Mechanics ◽

Mechanics Analysis ◽

Out Of Plane ◽

Plane Bending ◽

Chain Links ◽

Mooring Chain

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Out-Of-Plane Bending Stiffnesses in Offshore Mooring Chain Links Based on Conventional and Advanced Numerical Simulation Techniques

Journal of Ocean Engineering and Technology ◽

10.26748/ksoe.2018.6.32.5.297 ◽

2018 ◽

Vol 32 (5) ◽

pp. 297-309

Author(s):

Joonmo Choung ◽

Jae-bin Lee ◽

Young Hun Kim

Keyword(s):

Numerical Simulation ◽

Simulation Techniques ◽

Out Of Plane ◽

Plane Bending ◽

Chain Links ◽

Mooring Chain

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Mooring Fatigue Assessment Evaluating Chain Twist and Out-of-Plane Bending for a Semi-submersible

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4046856 ◽

2020 ◽

Vol 142 (6) ◽

Author(s):

Xutian Xue ◽

Nian-Zhong Chen ◽

Yongchang Pu

Keyword(s):

Fracture Mechanics ◽

Comparative Study ◽

Mooring System ◽

Safety Factors ◽

Fatigue Assessment ◽

Pure Tension ◽

Out Of Plane ◽

Plane Bending ◽

Chain Links ◽

Mooring Chain

Abstract A mooring fatigue assessment for mooring chain links of a semi-submersible in Offshore West Africa (OWA) is presented. Three cases that mooring chain links are subjected to pure tension, out-of-plane bending (OPB), and torque are considered in the assessment. For the case that mooring chain links are subjected to pure tension, a comparative study on S–N curves, T–N curves, and fracture mechanics (FM)-based mooring system fatigue analyses is made, and the results show that the fatigue lives predicted by these three approaches are generally comparable if the safety factors suggested by API and DNVGL are applied to T–N curves and S–N curves based approaches. For the cases that mooring chain links are subjected to the OPB and torque, the investigation shows that fatigue lives of mooring chain links are decreased significantly due to the OPB effects, while the decline of fatigue lives of mooring chain links happens when the twist angles are more than 10 deg.

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Chain Out of Plane Bending (OPB) Fatigue Joint Industry Project (JIP) FEA Results and Multiaxiality Study Results

Volume 1: Offshore Technology; Offshore Geotechnics ◽

10.1115/omae2016-54198 ◽

2016 ◽

Cited By ~ 2

Author(s):

Lucile Rampi ◽

Andre Bignonnet ◽

Cedric Le Cunff ◽

Francois Bourgin ◽

Pedro Vargas

Keyword(s):

Fatigue Loading ◽

Time Frame ◽

Small Time ◽

Element Analysis ◽

Study Results ◽

Out Of Plane ◽

Bending Stresses ◽

Plane Bending ◽

Mooring Chain ◽

Fatigue Mechanism

In 2002, several mooring chains of a deepwater offloading buoy failed prematurely within a very small time frame. These chains were designed according to conventional offshore fatigue assessment using API recommendations. With this first deepwater buoy application, a new mooring chain fatigue mechanism was discovered. High pretension levels combined with significant mooring chain motions caused interlink rotations that generated significant Out of Plane Bending (OPB) fatigue loading. Traditionally, interlink rotations are relatively harmless and generate low bending stresses in the chain links. The intimate mating contact that occurs due to the plastic deformation during the proof loading and the high pretension of the more contemporary mooring designs have been identified as aggravating factors for this phenomenon. A Joint Industry Project (JIP), gathering 26 different companies, was started in 2007 to better understand the Out of Plane Bending (OPB) mooring chain fatigue mechanism and to propose mooring chain fatigue design recommendations. This paper summarizes the computational Finite Element Analysis (FEA) scope of work that provided the understanding and validation of the OPB mechanism through correlation with the test program results on chains. In addition, a multiaxial assessment of the fatigue stresses is studied and the main results are presented in this paper.

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Calculation Methodology of Out of Plane Bending of Mooring Chains

Volume 4: Materials Technology; Ocean Engineering ◽

10.1115/omae2007-29178 ◽

2007 ◽

Author(s):

E. ter Brake ◽

J. van der Cammen ◽

R. Uittenbogaard

Keyword(s):

Fatigue Damage ◽

Bending Moment ◽

Local Stress ◽

Mooring Line ◽

Empirical Formulas ◽

Out Of Plane ◽

Plane Bending ◽

Chain Links ◽

Mooring Chain

Recently, the phenomenon of out-of-plane bending (OPB) fatigue of mooring chain links emerged as an important parameter in the fatigue assessment of mooring lines. Vessel motions induce a bending moment at the top chain of a mooring line. This bending moment induces alternating local stresses in the link and thus contributes to fatigue damage of those links. High pretension mooring systems are particularly sensitive to this phenomenon, since a small vessel motion combined with a high tension results in a relatively large bending moment in the upper mooring chain links. In mooring systems with high pre-tensions, this damage is of much greater magnitude than the fatigue damage induced by tension-tension loading only. An extensive study has been executed to investigate the fatigue life of mooring chain in deep water systems. This paper presents the calculation procedure to include the effects of local chain bending in the overall mooring line fatigue analysis. It was concluded that despite the complexity of the OPB issue, it is a phenomenon that can be incorporated in the mooring analyses by means of numerical procedures. The developed method is based on extensive Finite Element Method (FEM) analyses of chain links. Models of multiple chain links have been used that take into account the plastic-elastic properties of the material and contact friction between chain links. The FE models are used to derive empirical relations, between load angles, interlink angles, bending moments and stresses. These calculations were made for different combinations of line tension, interlink friction and chain size. The results were stored in a database to gain insight in the out-of-plane bending phenomenon. This database provides empirical formulas to lead to the local stress in different points on a chain link. These empirical formulas are used to translate floater (vessel or buoy) motions into local stress variations and fatigue damages in chain links. The long-term motion behaviour of the floater is known, the long term tension and bending stress ranges can be obtained and thus a fatigue damage of the chain links can be calculated.

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Risk-Based Inspection Planning for Mooring Chain

21st International Conference on Offshore Mechanics and Arctic Engineering, Volume 2 ◽

10.1115/omae2002-28409 ◽

2002 ◽

Author(s):

Jan Mathisen ◽

Kjell Larsen

Keyword(s):

Probabilistic Modelling ◽

Structural Components ◽

Inspection Planning ◽

Single Chain ◽

Chain Link ◽

Northern North Sea ◽

A Chain ◽

Series Systems ◽

Chain Links ◽

Mooring Chain

The use of risk-based inspection planning for offshore structural components is becoming quite familiar. This paper describes an application of this technique to mooring chain. In many cases, the technique is based on probabilistic modelling of fatigue crack growth in the structural components, and updating of the failure probability on the basis of inspections. The extension of this basis from a single component to very many components is necessary to tackle series systems, such as mooring chain, where a fatigue fracture can arise in any chain link. The theoretical basis for the analysis is described, including details of the model for stochastic dependency between the chain links. Results are shown that compare failure probabilities for a single chain link and a chain segment. The effects of various levels of inspection coverage are illustrated. An example of a cost optimal inspection plan is developed for mooring chain on an FPSO in the northern North Sea.

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